This invention relates to an optical fiber strain measurement apparatus and an optical fiber strain measurement method using Brillouin scattered light.
With the evolution of optical fiber communication, distributed optical fiber sensing, in which the optical fiber itself serves as a sensing medium, has become an active area of research. Representative distributed optical fiber sensing is optical time domain reflectometry (OTDR), in which optical pulses are incident on an optical fiber from one end of the optical fiber, and light backscattered within the optical fiber is measured with respect to time. Backscattering in an optical fiber includes Rayleigh scattering, Brillouin scattering, and Raman scattering. Among others, OTDR that measures spontaneous Brillouin scattering is referred to as Brillouin OTDR (BOTDR) (see, for example, K. Koizumi, et al., “High-Speed Distributed Strain Measurement using Brillouin Optical Time-Domain Reflectometry Based-on Self-Delayed Heterodyne Detection”, ECOC 2015, P.1.07, Sep. 2015).
Brillouin scattering is observed at positions with the frequency shift of the order of GHz on Stokes and anti-Stokes sides with respect to the center frequency of the optical pulse incident on the optical fiber. The spectrum of Brillouin scattering is referred to as the Brillouin gain spectrum (BGS). The frequency shift and the spectral line width of the BGS are referred to as Brillouin frequency shift (BFS) and Brillouin line width, respectively. The BFS and the Brillouin line width vary depending on the material of the optical fiber and the wavelength of incident light. For example, in the case of a silica-based single-mode optical fiber, it is reported that the magnitude of the BFS and the Brillouin line width for a wavelength of 1.55 μm are approximately 11 GHz and approximately 30 MHz, respectively.
In addition, it is known that the BFS linearly changes at a rate of 500 MHz/% with respect to strain of an optical fiber. If this is converted into tensile strain and temperature, the tensile strain and the temperature respectively reach change amounts of 0.058 MHz/με and 1.18 MHz/° C. In BOTDR, it is therefore possible to measure changes in strain and temperature with respect to the longitudinal direction of an optical fiber. BOTDR has been attracting attention because BOTDR is usable for the purpose of monitoring large constructions represented by bridges and tunnels.
With reference to FIG. 5, a conventional optical fiber strain measurement apparatus will be described. FIG. 5 is a schematic block diagram illustrating the conventional optical fiber strain measurement apparatus.
A transmission unit 110 generates an optical pulse as probe light. The optical pulse generated by the transmission unit 110 is sent, via an optical circulator 20, to an optical fiber (measurement target optical fiber) 100 that is a measurement target. The backscattered light from the measurement target optical fiber 100 is sent to the receiver unit 30 via the optical circulator 20.
A receiver unit 30 includes a receiver-side optical bandpass filter (BPF) 32 and a self-delayed heterodyne interferometer 40. The self-delayed heterodyne interferometer 40 includes a splitter 42, an optical frequency shifter unit 43, a delay controller 48, a polarization controller 46, a multiplexer 50, a photodetector 60, a local oscillator 83, and a phase comparator 70.
The local oscillator 83 generates an electrical signal having a frequency fAOM.
The splitter 42 receives, via the receiver-side optical BPF 32, and splits the Stokes component of Brillouin backscattered light, which is caused by the probe light in the measurement target optical fiber 100, into the two branches of a first light path and a second light path.
The optical frequency shifter unit 43 is provided in the first light path. The optical frequency shifter unit 43 uses the electrical signal having the frequency fAOM generated by the local oscillator 83 to provide a frequency shift of the frequency fAOM to the light propagating through the first light path. In addition, the delay controller 48 and the polarization controller 46 are provided in the second light path. The delay controller 48 provides a delay time ti to the light propagating through the second light path. The polarization controller 46 controls the polarization of the light propagating through the second light path.
The multiplexer 50 multiplexes the light propagating through the first light path and the light propagating through the second light path to generate multiplexed light.
The photodetector 60 performs heterodyne detection on the multiplexed light to generate a beat signal. The beat signal generated by the photodetector 60 is sent to the phase comparator 70 as a first electrical signal. In addition, an electrical signal generated by the local oscillator 83 is sent as a second electrical signal to the phase comparator 70.
The phase comparator 70 performs homodyne detection on the first electrical signal and the second electrical signal to generate a homodyne signal. Here, the first and second electrical signals are each a beat signal having the frequency fAOM, so that homodyne detection on these electrical signals allows a change of 2πfbτ to be output as a phase difference. Here, fb represents the optical frequency of spontaneous Brillouin scattered light.
The transmission unit 110 of the conventional optical fiber strain measurement apparatus generates a rectangular optical pulse that is input into the measurement target optical fiber 100. To generate this optical pulse, the transmission unit 110 includes, for example, a continuous light source 112 which generates continuous light, and a modulator 114 that modulates this continuous light with an electrical pulse.